To me, it is not about "can Stackoverflow already fill the need for an emacs sx?", but rather "does it?". And this question is not about "where can we find answers about emacs?", it is about "where can we ask questions about emacs?".
emacs tag on stackoverflow is running at ~10 questions per day. It would be within the "okay" range for a beta website, but given the surge of interest for this very beta, I wouldn't be surprised if we ended with way more than these ~10 questions.
Note also that it is very rarely that emacs questions to google lead back to stackoverflow (or any other stackexchange site). Instead you have blogs, you have emacswiki, you have scattered wikis on github, and you have gmane (which is underrepresented on google as well, though).
So this leads me to believe that there is a need for an emacs Q/A website. Stackoverflow, superuser, etc. may be technically able to fulfill that need, but apparently they don't (otherwise, there wouldn't be a need anymore, eh?).
So the question is "why aren't there more emacs-related questions on SO?" I do not have a definite answer to that, but a few points were already mentionned around the place:
- people may prefer asking questions in a more community-like environment (hell, there are questions on emacswiki ! ), and you can't really build a community around a tag;
- SO is really intimidating. The amount of questions, the pace at which they appear and disappear, the very technical appearance of the questions on the first page... It makes it very hard to dive in for newcomers, especially with questions which wouldn't look quite right (how many emacs questions do you see on the front page? ~10 questions a day is nothing for SO!);
- not all emacs users are professional programmers. Some are amateur programmers, some are researchers in various fields... The same way TeX is trying to appeal to more than just programmers and mathematicians, a lot of people could benefit from using emacs. Students taking notes with MS Word could use org-mode, no matter if it is a math or a history class! Do you really see a history student asking a question on SO?
The second of these problems could be solved with appropriate information and "marketing", of the form "questions like XXX are also welcome on SO". But then a lot of other underrepresented communities would rightfully be annoyed that they don't get this kind of publicity as well (but most of these communities could not populate an independent SX site of their own). And the other problems would remain unsolved.
Several people have already made a parallel with tex.sx vs SO and askubuntu vs UX. This parallel extends to the three problems I have stated above. And as far as I know, both these sites ended up nicely complementing the set of advice that the older, bigger websites could provide, and, most importantly, they have managed to fill the niche left by the third problem: beginners are welcome and feel welcome.
Tl;dr: An emacs.sx website will not divert much traffic/questions/answers from SO and other sites, because this traffic is already diverted by a dozen blogs, by emacswiki and by gmane. Yet apparently a lot of SX-ers would be willing to use an emacs.sx website.